Heavy freezing drizzle can cause extensive engine damage in jets waiting for takeoff. Often, freezing drizzle can appear to be only harmless light drizzle to even trained meteorologists, with the result that preventative or remedial de-icing is not done. The National Center for Atmospheric Research reported that two cases of heavy freezing drizzle at Denver International Airport on Oct. 31, 2002, and the same date in 2003, caused a total of $2.85 million in damage to 18 jet engines on United Airlines 737 aircraft.